Referring to FIG. 1, it is a structural principle diagram of a conventional fingerprint image acquisition device. Specifically, the fingerprint image acquisition device shown in FIG. 1 includes: a light source 12, a prism 14, a convex lens 16 and an image sensor 18. The prism 14 has a contacting surface for a finger 10. A light beam from the light source 12 enters into the prism 14 through a side of the prism 14, the light beam in the prism 14 moves towards the contacting surface for the finger 10 and is reflected to another side of the prism 14 by a fingerprint image of the finger 10; after that the light beam is focused on the image sensor 18 by going through the convex lens 16, then an appropriate signal processing is performed so as to achieve fingerprint recognition.
However, as the light source 12, the convex lens 16 and the image sensor 18 are located at different sides of the prism 14, a space just below the finger contacting surface of the prism 14 cannot be utilized, as a result, a volume of the fingerprint image acquisition device cannot be decreased.